Studies in the Calcitc Group. 215 



that meet at that point and consequently only one eight of each 

 belongs to this particular unit. In the same way the six atoms 

 lying in the middle of the faces of the rhombohedron belong to 

 two adjacent units and therefore only one half of each goes with 

 this cell. From this is is seen that only four calcium atoms with 

 their attendant carbon and oxygen atoms belong to this volume. 



The measurements given in the case of dolomite, siderite and 

 rhodochrosite show that their molecular arrangement is of the 

 same sort as with calcite. No measurements are given for mag- 

 nesite, probably because of the difficulty of securing suitable 

 specimens of that mineral. There can be no doubt, however, 

 but that its structure is similar to that of the other members of 

 the group. 



Consequently, if we have given the specific gravities for the 

 different members of this group, it is possible to calculate the 

 volumes of the unit cells of their crystal lattices. The different 

 specific gravities were taken as follows: calcite 2.715, dolomite 

 2.87, magnesite 2.96, rhodochrosite 3.70 and siderite 3.89. In 

 the case of calcite the specific gravity was obtained from meas- 

 urements on Iceland spar which is practically pure CaCOg. In 

 the case of the other members of the group, which do not occur 

 in such pure form, the numbers were derived from a study of the 

 different analyses of the minerals with which specific gravity 

 determinations were given. As will be shown later, these figures 

 have been proven to be at least very closely correct. The cell 

 selected for the calculation was the same as that showm in Fig. 

 I, or eight times the volume of the unit cell. The formula used 



1 1 • 1.64 X io~^'* X mol. wt. ^, 1 -ri 



m the calculation was — X 4 = vol. i he 



sp.g. 



results are given in the table below. In the first column are 



given the actual volumes obtained, while in the second column, 



for the sake of greater ease in comparison, the volume of the 



calcite cell is reduced to 100 and the volumes of the other cells 



are expressed on the same scale. Of course the molecular volumes 



for the different minerals, obtained in the usual way by dividing 



the molecular weight by specific gravity, would have given 



results proportionate to these. It seemed interesting, however, 



to ffive the actual figures for a definite cell of the crvstal lattice. 



